Rotor for brushless DC motor and brushless DC motor

Abstract

A rotor unit for a brushless direct current motor (BLDC motor) comprises a main body integrally formed with a rotor shaft and an annular ring magnet mounted on the main body. The annular magnet comprises an anisotropic hard ferrite and is laterally magnetized. The rotary shaft and the main body of the rotor unit may be formed from one plastic material. The rotor unit is supported within the BLDC motor via self-lubricating bearing seats.

Description

BACKGROUND[0001]1. Technical Field[0002]The present invention relates to the field of brushless direct current electric motors, called BLDC motors, in particular for use in wet-running circulation pumps of home appliances, for example. In particular, the invention relates to a rotor unit for such a BLDC motor according to the preamble of independent claim 1.[0003]2. Description of the Related Art[0004]EP 1 841 041 A1 shows a BLDC motor for use in wet-running pumps. The BLDC motor has a permanently excited rotor which comprises a plastic connection between a reflux base and a rotor shaft. The plastic connection is formed by injection molding and provides for a fixed and watertight attachment of the reflux base to the rotor shaft. Furthermore, permanent magnets are attached to the reflux base via another plastic connection. Both plastic connections may be formed in the same injection molding process. Additionally, the rotor is provided with an injection molded plastic sheath enclosing...

AI Technical Summary

Benefits of technology

The invention provides an improved rotor unit for a brushless direct current motor that is easier to produce and reduces weight and torque inertia of the motor. The rotor unit comprises a main body with a cylindrical portion integrally formed with a rotor shaft, which simplifies manufacturing and reduces costs for use in home appliances. The main body may be made of plastic material for better abrasive wear resistance, which further improves the motor's operating characteristics and service life. The rotor unit also uses an annular magnet with alternating magnetic poles, which results in a more ideal sinus-shaped magnetic flux distribution and reduces torque variations and acoustic emissions.

Problems solved by technology

However, rare-earth materials are very expensive and suffer under shortage of resources.

Moreover, the materials are susceptible to corrosion in aqueous dilutions so that it is necessary to completely coat and mold the magnets for preventing corrosion.

As another disadvantage of the rotor of EP 1 841 041 A1, it is required to overmold the rotor components two times, namely once for the cage or reflux base and once for the outside overmolding to mount the individual magnet elements.

Another disadvantage of the rotor shown in this document is that the stop disc of the bearing has to be mounted together with an o-seal in an additional production step.

However, anisotropic ferrites exhibit a relative low magnetic remanence (usually less than 0.5 Tesla), and magnetizing the core has to be performed in a sheathed condition with a magnetic gap to the magnetizing device resulting in a non-optimal utilization of material.

Furthermore, the mounting process is laborious with respect to the core laminate for the magnetic reflux and with respect to sheathing and positioning of the magnet elements.

Usage of steel for the shaft represents a tradeoff between high hardness, abrasive wear and corrosion resistance in brine, so that the material characteristics of the rotor shaft are not optimal.

This leads to wear of the rotor shaft resulting in slackness and acoustic emissions.

Furthermore, the whole construction of these rotors in connection with the used materials results in magnetic and mechanic imbalances because of unavoidable tolerances during manufacturing.

Thus, also the induction distribution is not ideal and deviates from a sinus shape resulting in increased torque variations and further acoustic emissions.

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